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Description:

pystatemachine 1.2

pystatemachine
pystatemachine is a versatile, yet easy-to-use finite-state machine
library written in python. It provides functions to turn any python
object into a finite-state automaton which changes from one State to
another when initiated by a triggering event.

Usage
A finite-state machine is defined by a list of its states, and the
triggering condition for each transition. pystatemachine offers an
event decorator for a classes’ bound methods, a State class to
define the finite-state machine’s states, and a
acts_as_state_machine decorator for turning any python (new- or
old-style) class into a finite-state machine. By default, any
event-decorated method may raise errors. Optionally, a
transition_failure_handler decorator turns any class method into a
failure handler which gets invoked when an event-decorated method
raises an error.

Example
Following, a turnstile is modeled.

An example of a very simple mechanism that can be modeled by a state
machine is a turnstile. A turnstile is a gate with three rotating
arms at waist height, one across the entryway. Initially the arms
are locked, barring the entry, preventing customers from passing
through. Depositing a coin or token in a slot on the turnstile
unlocks the arms, allowing a single customer to push through. After
the customer passes through, the arms are locked again until another
coin is inserted. - from [Wikipedia]
(http://en.wikipedia.org/wiki/Finite-state_machine#Example:_a_turnstile)

@acts_as_state_machine
class Turnstile(object):
locked = State('locked', initial=True)
unlocked = State('unlocked')

@event(from_states=(locked, unlocked), to_state=unlocked)
def coin(self):
assert random.random() > .5, 'failing for demonstration purposes, only ..'
print('*blingbling* .. unlocked!')

@event(from_states=(locked, unlocked), to_state=locked)
def push(self):
print('*push* .. locked!')

@transition_failure_handler(calling_sequence=2)
def turnstile_malfunction(self, method, from_state, to_state, error):
print('state transition from {0.name} to {1.name} failed. Reason: {2}'.format(from_state, to_state, error))

@transition_failure_handler(calling_sequence=1)
def before_turnstile_malfunction(self, method, from_state, to_state, error):
print('before state transition failure handler ..')

import random

turnstile = Turnstile()
for _ in range(10):
handler = random.choice([turnstile.coin, turnstile.push])
handler()

Changelog

1.2

exceptions in an event-decorated function are now reraised when no
transition failure handler was registered

1.1

added a decorator for registering a class’ method as exception
handler when an ‘event’-decorated method fails. multiple methods may
be registered as transition failure handler: they are invoked in the
order given by the optional ‘calling_sequence’ keyword

1.0

first public release

License
pystatemachine is available under MIT
License.

Download
You can download
pystatemachine.py.
Alternatively:
git clone git@github.com:cmaugg/pystatemachine